FIG. 5 is a block diagram showing an example of a conventional power unit constituting a chopper boost regulator including a power line. An input primary voltage from the DC power source 100 is inputted into a switching converter 110.
The switching converter 110 is composed of a switching element 111, a coil 112, a diode 113, and smoothing capacitors 114 and 115. Due to the voltage inputted into the switching converter 110, energy is charged in the coil 112 while the switching element 111 is turned on, and the charged energy is released via the diode 113 while the switching element 111 is turned off. When releasing the energy, this energy is added on the input voltage and causes boosting action. The output of the switching converter 110 is converted into a predetermined alternating current by an inverter 120 and becomes an output of the power unit.
The switching converter of such a power unit is frequently used for maintaining the input voltage of the inverter stable when the DC power source 100 is power DC converted from power generated in an engine generator or is a small-sized fuel cell and the output capacity thereof is comparatively not very high.
Such a switching converter 110 is subjected to the following feedback control for maintaining its output voltage stable. An error amplifier 130 amplifies the deviation between an output voltage Vout of the switching converter 110 and a reference voltage Vref, and a comparator 131 compares this deviation with a predetermined allowable error. Then, a duty ratio (ON time duty ratio) is determined so that the deviation comes in under the allowable error, and according to this duty ratio, the switching element 111 is driven by a switching part 132.
In this duty control, when a protection circuit for the switching element 111 is provided, this protection circuit is required to have a function of protection from an overcurrent while maintaining the output voltage at a predetermined value, that is, without stopping the output. Therefore, not a voltage detection type short-circuit protection circuit but a circuit that limits the operation of the switching element 111 by detecting the energization state of the switching element 111 is needed. Accordingly, in the example of FIG. 5, a current detector 133 and a limiter 134 for limiting the operation of the switching element 111 are provided. The example of the switching converter is disclosed in Japanese Published Unexamined Patent Application Nos. 2002-64976 and 2004-40858, etc.
Patent document 1: JP 2002-64976 A
Patent document 2: JP 2004-40858 A